1. Technical Field of the Invention
The present invention relates to a turbofan engine which has a high bypass ratio and can achieve a good mileage and a low noise, and more particularly to a dovetail structure of a fan in which an inlet hub diameter is smaller than an outlet hub diameter.
2. Description of the Related Art
FIG. 1 is a schematic view of an aircraft engine 51 (a turbojet engine). As shown in this drawing, the turbojet engine is provided with a fan 52 taking in an air, a compressor 53 compressing the intake air, a burning device 54 burning a fuel by the compressed air, a turbine 55 driving the fan 52 and the compressor 53 on the basis of a combustion gas of the burning device 54, an afterburner 56 afterburning for increasing a thrust, and the like.
The afterburner 56 is constituted by a flame holder 57 having a triangular cross section or the like and forming a circulating region in a downstream side so as to achieve a flame holding, a fuel nozzle 58 for jetting out a fuel, a spark plug 59 and the like, jets out from an exhaust nozzle 62 through an inner side of a liner 61 in an inner side of an after duct 60, and increases a thrust.
In the turbojet engine mentioned above, a structure in which the fan 52 taking in the air is enlarged in size, and a bypass ratio is enlarged is called “turbofan engine”. The bypass ratio corresponds to a flow ratio (bypass flow/core flow) of a bypass flow bypassing a core engine (the compressor 53, the burning device 54 and the turbine 55 mentioned above) with respect an air flow (a core flow) flowing into the core engine. The larger the bypass ratio is, the more the flow rate of the exhaust jet is reduced, so that there is obtained an effect of lowering a noise and a specific fuel consumption.
However, in the turbo engine mentioned above, if the bypass ratio is enlarged, a fan first stage rotor blade (a fan blade in the front row) and an inner diameter of a casing surrounding it become enlarged, and there is a problem that a weight of the engine is increased.
In other words, since a fan first stage rotor blade 52a having a structure embedded in a spinner 63 of the turbofan engine has an embedded structure, a certain degree of hub/tip ratio (inlet hub diameter/tip diameter shown in FIG. 2: normally about 0.3) is necessary, and a fan inlet area becomes narrower at an area corresponding to the inlet hub diameter.
Accordingly, if it is intended to increase the bypass ratio in order to achieve the good mileage and the low noise, the fan diameter and the inlet hub diameter become further larger, and the weight of the engine is increased.
Then, in order to solve the problem, the same applicant as that of the present invention has already proposed “turbofan engine” in patent document 1.
The turbofan engine is provided with a fan first stage rotor blade 65 for taking in an air, and a spinner 64 rotationally driving the fan first stage rotor blade, as shown in FIG. 3, and the spinner has a spiral blade 66 extending spirally to an outer side in a radial direction from an axis thereof and sucking the air from a front surface of the spinner so as to supply to the fan first stage rotor blade.
In this case, reference numerals 67 and 67′ denote a casing inner diameter, and reference numeral 68 denotes an inflow air flow.
In accordance with the structure of the patent document 1, since the spinner 64 has the spiral blade 66 extending spirally to the outer side in the radial direction from the axis thereof and sucking the air from the front surface of the spinner so as to supply to the fan first stage rotor blade 65, it is possible to suck the air from the front surface of the spinner corresponding to the inlet hub diameter so as to compress the air and supply to the fan first stage rotor blade 65.
Therefore, since an entire area in the front side of the engine becomes the air inflow area of the fan first stage rotor blade 65, it is possible to make the fan diameter small, and it is possible to reduce the engine weight.
Further, since the fan first stage rotor blade 65 and the spiral blade 66 of the turbofan engine mentioned above are integrally coupled, it is possible to connect the respective blade surfaces smoothly, and it is possible to suck and compress the air efficiently. Hereinafter, the fan in which the fan first stage rotor blade 65 and the spiral blade 66 are integrally formed, the air can be sucked from the front surface of the spinner, and the substantial hub/tip ratio can be set to 0 is called as “zero hub tip ratio fan”.
Patent Document 1: Japanese Unexamined Patent Publication No. 2004-27854, “TURBOFAN ENGINE”
Patent Document 2: U.S. Pat. No. 6,764,282, “BLADE FOR TURBINE ENGINE”
It is necessary to attach the fan blade of the turbofan engine to a portion around a discoid disc (or spinner) rotationally driven by a turbine. Accordingly, in conventional, there has been generally employed a dovetail structure in which a dovetail portion extending in a longitudinal direction is provided in a root portion of the fan blade, and the dovetail portion is fitted to a dovetail groove formed around the disc.
In the conventional dovetail structure mentioned above, the dovetail portion and the dovetail groove are provided in parallel to a rotation axis Z-Z of the disc, thereby preventing a centrifugal force applied to the fan blade from generating a component force in an axial direction. Hereinafter, this structure is called as “parallel dovetail structure”.
However, in the case that a diameter change in an inner side of a donut-shaped flow path to which the fan blade is attached is large, if the parallel dovetail structure is employed, it is necessary to make a diameter of the dovetail portion and the dovetail groove equal to or smaller than a minimum diameter of the flow path, and there is a risk that a stress generated in the dovetail portion and the dovetail groove becomes too large.
Accordingly, there has been proposed a dovetail structure in which the dovetail portion and the dovetail groove shown in FIG. 4 are sloped with respect to the rotation axis (for example, patent document 2). In this drawing, reference numeral 71 denotes a disc, reference numeral 73 denotes a blade, reference numeral 77 denotes a dovetail, and reference numeral 79 denotes a tab.
Hereinafter, this structure is called as “slope dovetail structure”.
However, in the case of the zero hub tip ratio fan mentioned above, since the hub/tip ratio is between 0 and 0.35, and the diameter of the inner side of the donut-shaped flow path to which the zero hub tip ratio fan is attached is zero or close to zero, there is a problem that the parallel dovetail structure can not be essentially applied.
Further, even in the case that the slope dovetail structure is applied, it is impossible to support the centrifugal force of the front side portion (the portion corresponding to the spiral blade mentioned above) of the zero hub tip ratio fan by the disc (or the spinner).
Further, in the case that the slope dovetail structure is applied to the zero hub tip ratio fan, since the component force in the axial direction of the centrifugal force applied to the fan blade is large, there is a risk that the generated stress becomes too large in the structure having a small shear area such as the tab disclosed in the patent document 2.